9.52.2.2 Register Names

The assembly syntax for a register file entry is the “short” name for
a TIE register file followed by the index into that register file. For
example, the general-purpose AR register file has a short name of
a, so these registers are named a0...a15.
As a special feature, sp is also supported as a synonym for
a1. Additional registers may be added by processor configuration
options and by designer-defined TIE extensions. An initial `$'
character is optional in all register names.